One major wall I’ve been hitting with this prequel fic is that the crossovers in question have similar universes, but wildly different takes on morality. For example - Supernatural treats a possessed person (or ‘meat suit’) as expendable, when in the Buffy-verse their safety would be the goal of the hunt. How would those two mentalities clash if they came into contact with one another?
I’m assuming that the Winchesters and Buffy’s group would be only too happy to lecture on the morality and ethics of their own universe, but that’s something that happens after someone’s died or worse.
Would the fighting styles get in each other’s way? Would they drop everything in the middle of a major fight (as happens so often on both shows) to long-wind the other party over doing something ‘wrong’? Would they end up helping or hurting each other in the long run? Are these world views the end result of a world full of powerful women vs. a world full of powerful men? All things to really consider before committing concepts to ‘paper’, so to speak.